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Hemlock’s Existential Crisis

A couple months ago, I had the opportunity to witness a salvage operation of a particularly beleaguered stand of eastern hemlock in southern New Hampshire. I have seen many harvest operations, and this one, aside from being conducted by chainsaw and cable skidder, was no different. Nonetheless, I felt a tinge of sadness as I watched. Around me were beautiful, centuries-old hemlocks that had succumbed to infestation of both elongate hemlock scale (EHS) and hemlock woolly adelgid (HWA). It was hard to notice anything particularly troubling by glancing through the stand—initially, it seemed relatively healthy—but all doubts were erased with the cracking howl of the first tree felled.

As the skidder pulled the trees to the landing, seeing the minty-white foliage was a dead give-away of the adelgid’s sizable damage. The insect’s woolly namesake gave the tree the appearance of holding fresh snow, which was an out-of-place sight on this unusually warm December day. The normally dark-green needles were flecked with the brown bodies of the EHS. Though unnoticeable from afar, it gave the needles a sickly appearance. A more thorough investigation of the stand further revealed a tertiary infestation of opportunistic hemlock borers. Trees were encircled by piles of fresh orange mulch—the chips resulting from woodpeckers feeding on these insects who drilled their way into the cambium. If the EHS and HWA didn’t kill the tree, the borer infestations surely did.

Needles of the eastern hemlock. The HWA’s wool can be seen at the needle bases, and the brown spots on the needles are the bodies of the EHS

A Slow Death

The presence of these two insects is a challenge, and it’s been a challenge since 1999 when the hemlock woolly adelgid first arrived within state borders. Elongate hemlock scale is a more recent pest, first found in Maine in 2010. The adelgid, deemed “woolly” because of the white fluff it covers itself in, secures itself to the base of the needles and sucks the sap out of the twig. The elongate hemlock scale, bearing the appearance of a waxy bump on the needle, attaches itself to the foliage and likewise feeds from it. The feeding habits of both insects defoliate the tree and deplete it of its energy. Subsequent death of the twigs and buds prevents further growth from replenishing the tree. Slowly, the crown thins from the bottom up, and left unchecked, mortality ensues.

Though these species are found on trees singularly (in Maine, HWA is the more populous of the two), they tend to play off each other and can be found together. The stress one species puts on a tree seems to draw or exacerbate the presence of another. Additionally, they are both spread primarily by wind and birds, so if a stand has an outbreak of one species, it is likely susceptible to the same transmission agents of the counterpart. In the unfortunate cases when they are found together, mortality is only hastened. The stress they put on the tree can then catalyze the infestation of other usually-benign native pests, such as the hemlock borer. When that is the case, it is certainly the death knell—a feedback loop of terror.

Sadly, there is currently little that can be done once the insects are established. Insecticidal treatments exist, but they are not economic on a large scale. Probably the best hope we have in containing the spread is by the propagation and release of predatory insects, namely the Japanese Sasajiscymnus tsugae and the western Laricobius nigrinus, both of which have been released by the Maine Forest Service. In the meantime, it is important for everyone to know the extent of HWA’s presence and be aware of how to identify the pest and slow its spread, information on which can be found here. Material on EHS can be found here.

Maine.gov

What Will Become of Hemlock?

This is not the first time the eastern hemlock has been threatened by an insect outbreak. In postglacial, prehistoric times, the eastern hemlock was a dominant tree in Maine. But 5,400 years ago, a hemlock looper outbreak greatly reduced populations. Two thousand years later, the population rebounded only to drop again around 500 A.D., likely as the result of modestly cooling temperatures. This gave us the spruce-fir forests that sealed Maine’s economic destiny. The timescales considered may be too large for practicality, but our forests are dynamic and always changing. Though introduced by man, these pests are nonetheless characters in the forest’s ever-unfolding story.

Presently, HWA and EHS are relegated to coastal Maine, but no one knows how far north they could spread or what impact they will ultimately have on eastern hemlock populations and diameter distributions. For the forest products industry, the answer is largely inconsequential—hemlock, to be blunt, is commercially useless. For biodiversity, wildlife, and riparian zone management, however, it is a question of the utmost importance. What happens next remains to be seen, as the next chapter of Maine’s forests has not yet been written. Hopefully, the scene from New Hampshire will be an anomaly—a minor setback in the annals of natural history. Maine’s beautiful hemlock stands, with luck and the work of landowners, foresters, and state agencies, will flourish for generations to come.